Lessons From Deep Space: Why Pioneer Is Important

NASA's Voyager spacecraft is not the only important deep-space mission to remember. The Pioneer spacecraft and the Pioneer Anomaly are part of a classic engineering story about good design.

"The Voyager spacecraft has left the solar system," read many headlines over the last few months. But its predecessor, the Pioneer spacecraft, should not be overshadowed. It left an important legacy as well, not for its planetary astronomy discoveries, but for its engineering lessons.

The pathfinding mission for the Voyager spacecraft's tour of the outer planets was the Pioneer mission, with Pioneer 10 launched on March 2, 1972. Just 19 months later, Pioneer 10 passed within 132,000 km of Jupiter, returning more than 500 images taken with its single-pixel camera.

Its primary mission was to explore Jupiter. In fact, Pioneer's discovery of the Jovian radiation belt, 1,000 times as intense as expected, came just in time to add radiation-hardened electronics to the still-under-construction Voyager craft -- electronics that are credited with enabling its long life.

Perhaps the part of Pioneer's mission most important to the engineering community, however, began only after it left Jupiter's influence.

The Pioneer 10 and 11 spacecrafts were the last crafts to have their long range radio dishes aimed back at earth by spin stabilization, rather than adjusted with thrusters or reaction gyro wheels. Between its roughly annual thrust corrections, the Pioneer 10 flew a ballistic trajectory, acting as a test mass to plot out the gravitational fields of the outer solar system.

A group at JPL, lead by John Anderson, had the idea of using the motion of Pioneer 10 as a high-precision probe of the gravitational environment of the solar system, searching for unknown planets and maybe even searching for low-frequency gravitational waves from small wobbles.

After all, this was how Neptune was discovered in 1846. Urbain Le Verrier, astronomer and mathematician, not willing to let go of the accuracy of the Newtonian Theory of Gravity, realized he could account for the well-documented anomalies in the motion of Uranus by postulating a hidden planet in a precise orbit. Neptune was found precisely where he predicted it to make Newtonian gravity work. As François Arago said, Le Verrier was the first astronomer to "find a planet with the point of his pen."

He was not so lucky in applying the same analysis to the motion of Mercury. To account for the well-documented 43 arc-second-per-century anomalous shift in the perihelion of Mercury, Le Verrier proposed the Planet Vulcan, in orbit much closer to the Sun than Mercury. Vulcan was never found, but this anomaly with Mercury was one of the three observations that confirmed general relatively, 70 years later.

This was the context in which Anderson's team set out to do precision celestial mechanics analysis of the motion of the Pioneer spacecraft.

My connection to Pioneer 10 was my job as computer operator at the University of Iowa Physics Research Center (PRC, now known as Van Allen Hall) in late '73.

Pioneer 10 was just going past Jupiter and they were in "high data rate" and my job consisted of mounting a tape, starting the job, and then finding something to do for the next 8 hours of my 3rd shift while the program generated 35mm film plots of the radiation and magnetometer readings.

The title photo on my web site http://www.rostenbach.com shows me (and me) in front of the Univac 418 computers that did the processing. The left hand computer, the UNIVAC 418 model II (18 bit unary math processor with 16 K [of 18 bit words] of core memory at ½ MHz) did the bulk of the processing and just to the left of it, in the far back corner is the Xerox microfilm plotter.

Thanks for posting this. Very interesting and very learning to read. I think these kind of articles are the ones for young people to go after a technical study rather than the alpha side. And oh do we need them very very hard.... (here in Europe)

That is one of the more fascinating aspects of all space travel to me right now. Our computational tech is changing so much faster than our ability to travel quickly that there is this massive gap created any time we travel far. They have to be proud of their work though, those things have functioned amazingly.

The designers of these craft must be so proud of how well they have done their job, and the amount of information gained from their missions quite apart from the actual data sent back. I've often wondered if they think, "What if these spacecraft had today's technology...?" It takes so long to get there that technology will have advanced considerably by the time they reach their destinations...